7Li NMR chemical shift titration and theoretical DFT calculation studies: solvent and anion effects on second‐order complexation of 12‐crown‐4 and 1‐aza‐12‐crown‐4 with Lithium cation in several aprotic solvents

⁷Li NMR titration was used to determine stepwise complexation constants for the second‐order complexation of lithium cation with 12‐crown‐4 in acetonitrile, propylene carbonate and a binary mixture of propylene carbonate and dimethyl carbonate. The anions used were perchlorate, hexaflurophosphate and trifluromethanesulfonate. A second ligand 1‐aza‐12‐crown‐4 was similarly investigated. The exchange between the free and complexed cation in these reactions is fast on an NMR timescale resulting in a single lithium peak which is a concentration‐weighted average of the free and bound species. Solvent effects show that the 1:1 complex is much more stable in acetonitrile than in propylene carbonate or in the propylene carbonate dimethyl carbonate mixture. Anion effects for a given solvent were small. Optimized geometries of the free ligands and the 1:1 and 1:2 (sandwich) metal–ligand complexes were predicted by hybrid‐density functional theory using the Gaussian 03 software package. Results were compared to literature values for 1:1 stability constants found by microcalorimetry for several of these systems and are found to be in good agreement. Although microcalorimetry only considered the formation of 1:1 complexes, ⁷Li NMR shows evidence that both 1:1 and 1:2 complexes should be considered. Copyright © 2009 John Wiley & Sons, Ltd.